Mobile networks are currently evolving from pure circuit switched (CS) networks towards IP based networks, and by that integrate into IP based infrastructures that are also used for the Internet, the World Wide Web and the datacom industry.
With this trend mobile networks closely follow the evolution steps of the wireline networks, where Voice over Internet Protocol (VoIP) via Digital Subscriber Line (DSL) access or via Wireless Local Area Network (WLAN) access is an existing technology today.
Mobile operators that install IP Multimedia Subsystem (IMS) networks and offer IMS services intend to make these services available also to Global Systems for Mobile Communications (GSM)/Wideband Code Division Multiple Access (WCDMA) subscribers. Hereto, calls from and to this group of GSM/WCDMA subscribers need to be routed through the IMS network, in order to reach an IMS service engine. This concept is called IMS Centralized Services (ICS). The IMS centralized Services work item in the 3rd Generation Partnership Program (3GPP) targets at using the circuit switched (CS) access for an access to IMS services (see, for example, 3GPP Technical Specification 23.292) and is complemented by IMS Service Continuity(see, for example, 3GPP Technical Specification 23.237).
In parallel to the ICS trend, the packet core is evolving (Evolved Packet Core, EPC) as part of the Evolved Packet System (EPS), supporting eUTRAN (evolved Universal Terrestrial Radio Access Network) as new Radio Access Network (RAN). As part of this discussion, work on Single Radio Voice Call Continuity (SRVCC) is ongoing in 3GPP SA2 (see, for example, 3GPP Technical Specification 23.216), enabling to transfer an IMS voice call from a EPS based (access) network towards a CS based (access) network and vice versa. It is envisioned that SRVCC is combined with ICS for an IMS centralized solution supporting also SRVCC from eUTRAN to UTRAN/GERAN.
Referring to FIG. 1, an exemplary SRVCC architecture figure for a handover of an user equipment from an eUTRAN access network to a to CS access network, particularly to the UTRAN/GERAN access networks, is illustrated:
An user equipment UE is attached to the eUTRAN access network (depicted as E-UTRAN) via an LTE-Uu interface. A Mobility Management Entity MME is in communication with the eUTRAN access network as well as with a Serving GPRS Support Node (SGSN) which in turn is in communication with the UTRAN/GERAN access networks. A Mobile Switching Center (MSC) Server is in communication with the target UTRAN/GERAN access networks allowing for circuit switched data transfer. The MSC Server is enhanced for SRVCC and may have an ISDN User Part (ISUP) or a Session Initiation Protocol (SIP) interface. In the following, it is assumed that the MSC Server offers a SIP interface.
It is currently discussed to expand the SRVCC solution described in the 3GPP Technical Specification 23.216 by a SGs reference point or interface between the MSC Server and a Mobility Management Entity (MME).
Referring to FIG. 2, an exemplary architecture of a telecommunications network for a handover of an user equipment from an e-UTRAN access network to UTRAN/GERAN access networks is illustrated.
The architecture shown in FIG. 2 is similar to the SRVCC architecture shown in FIG. 1. Further, a SGs interface between a MME and a MSC Server is depicted.
Referring to FIG. 3, an exemplary architecture of a telecommunications network for an user equipment attaching to UTRAN/GERAN access networks and to an eUTRAN access network (depicted as E-UTRAN) is illustrated. As also known from the 3GPP Technical Specification 23.272, there is a SGs reference point between the MSC Server and the MME.
It may be expected that in certain configurations of the architecture of a telecommunications network the MSC Server may have a Gs reference point or interface for communication with a SGSN and a SGs interface for communication with the MME. In both cases, the UE would be CS attached in the MSC Server also when camping on a PS access only. It may be noted that a CS attachment of the user equipment may be performed using, by way of example, the Gs and SGs interfaces which are currently under specification in 3GPP but show certain deficiencies. However, further interfaces may also be usable for attaching an user equipment to a MSC Server when the user equipment is camping on a PS access network. Hence, the SGs and Gs interfaces are used as exemplary embodiments of the invention but do not limit the scope of the invention.
Accordingly, when a MSC Server enhanced for ICS receives a request for a CS attachment from the UE, the MSC Server may register the user in IMS, as described in the 3GPP Technical Specification 23.292. It may be noted that the Technical Specification 23.292 does not discuss the case of performing a circuit switched attachment procedure of the UE via the SGs interface. Depending on the preferences, the SCC AS (Service Centralization and Continuity Application Server) may consider this registration in a Terminating Access Domain Selection (T-ADS) and/or a Serving-Call Session Control Function (S-CSCF) may try to perform sequential or parallel forking for terminating sessions to the user equipment using a MSC Server, even when a user equipment/a subscriber is only reachable via a packet switched access.
The SGs may be used in SRVCC to avoid interacting with the Home Subscriber Server (HSS)/Home Location Register (HLR) during performing a handover from a packet switched access such as the eUTRAN access/High Speed Packet Access (HSPA) towards a circuit switched (CS) access. It may take up to several seconds to download subscriber data from the HLR in a visited network. This delay may be unacceptable for SRVCC, however, the solution currently described in the 3GPP Technical Specification 23.216 may not make use of the SGs interface.